1. Field of the Invention
The present invention relates to a depletion-type NAND flash memory.
2. Description of the Related Art
A depletion-type NAND flash memory uses, as a memory cell, a depletion-type field effect transistor (FET) in which source/drain diffusion layers and a channel are of the same conductivity type (e.g., n-type). This depletion-type NAND flash memory has been attention recently owing to its suppressed short channel effect caused by shrinking (e.g., see Jpn. Pat. Appln. KOKAI Publication No. 11-163303).
this depletion-type NAND flash memory has been proposed several improvements.
For example, if a silicon-on-insulator (SOI) technique is applied to the depletion-type NAND flash memory, an SOI active layer where memory cells are formed is enclosed by an SOI insulating layer and an isolation insulating layer which are buried insulating layers. As a result, parasitic capacity generated in the memory cells is reduced.
Moreover, if the memory cell of the depletion-type NAND flash memory has a metal/oxide/nitride/oxide/silicon (MONOS) structure, manufacturing costs can be reduced, for example, by the simplification of the cell structure.
However, the depletion-type NAND flash memory has a peculiar problem. That is, turning on/off of the depletion-type FET as a memory cell is controlled by the generation/disappearance of the channel resulting from the expansion/contraction of width of a depletion layer. Then, the expansion/contraction of the width of the depletion layer depends on a charge quantity and a gate potential in a charge accumulation (storage) layer of the memory cell.
Therefore, if the pitch of the cells is reduced, the expansion/contraction of the depletion layer in the focus cell (memory cell) is affected by the charge quantity and the gate potential in the charge accumulation layer of the cell (memory cell) adjacent to the focus cell. This means that the charge quantity and the gate potential in the charge accumulation layer of the adjacent cell change the threshold of the focus cell during reading.
As a result, reading erroneous occurs in the case of reading, and programming erroneous occurs in the case of verify-reading.